1. Technical Field
The present invention relates to a semiconductor element, and more specifically to a vertical semiconductor element, such as a power diode, a power MOSFET and the like using a nitride compound semiconductor, and a method of manufacturing a vertical semiconductor element.
2. Related Arts
As semiconductor power device elements, there are such as a pn diode, a Schottky diode, a power MOSFET, an insulated gate bipolar transistor (IGBT), and the like. For example, regarding a performance required for the Schottky diode, for a forward current direction, a resistance is preferred to be as small as possible, and for a reverse current direction, the resistance is preferred to be as high as possible.
A nitride compound semiconductor represented by a GaN has a high-breakdown voltage, a preferable electron transport property and a preferable thermal conductivity, comparing to silicon. Therefore, it is expected for applying to a semiconductor power device using a nitride compound semiconductor, such as GaN or the like.
Furthermore, in the power device using the nitride compound semiconductor, an n-type layer is able to be thinner for obtaining the same breakdown voltage, comparing to the power device using a silicon. Thereby, the forward resistance becomes possible to be smaller.
For such nitride compound semiconductor power devices, there is disclosed a Schottky barrier diode in JP 2003-60212 (a published Japanese patent application: hereinafter, it is called as a document 1), which comprises a buffer layer formed by disposing an AlN layer and a GaN layer one after the other on a substrate of silicon, a GaN layer formed thereon, and an electrode formed on the GaN layer, which has a Schottky contact therewith.
Moreover, there is disclosed a Schottky barrier diode in JP 2004-031896 (hereinafter, it is called as a document 2), which comprises an n+-type GaN layer and an n-type GaN layer formed in order on a sapphire substrate, through a GaN buffer layer, a convex part formed by patterning the n-type GaN layer, and an AlGaN layer grown at both left and right sides of the convex part, and a two-dimensional electron gas is formed at a heterojunction area between the AlGaN layer and the n-type GaN layer. In this case, an anode electrode is connected to the convex part, and a cathode electrode is formed on the flat n+-type GaN layer at a side of the convex part.
Moreover, there is disclosed a Schottky barrier diode in JP 2006-156457 (hereinafter, it is called as a document 3), which comprises a GaN layer and an AlGaN layer formed in order on a silicon substrate, a Schottky electrode formed on the AlGaN layer, and a via formed through the AlGaN layer and the GaN layer to reach the silicon substrate, and an ohmic electrode, of which one part is implanted in the via.
Moreover, there is disclosed an epitaxial substrate in JP 2006-100801 (hereinafter, it is as a document 4), which comprises a GaN epitaxial film layer, of which thickness is more than or equal to 5 μm and less than or equal to 1000 μm, on a GaN substrate having a predetermined carrier density.
Moreover, there is disclosed a semiconductor element in JP 2006-310408 (hereinafter, it is described as a document 5), which comprises a GaN substrate having an electro-conductivity, a GaN region formed on the GaN substrate, and a Shottky electrode having a Schottky junction in the GaN region.
According to the Schottky barrier diode described in the above mentioned document 1, as the AlN layer exists in a current path, an on-resistance becomes high.
Moreover, according to the Schottky barrier diode disclosed in the above document 2, the on-resistance becomes lower by the two-dimensional electron gas. However, as the Schottky barrier diode is a flat type device in which the anode electrode and the cathode electrode are disposed laterally, an element area of the diode becomes large. Furthermore, when a distance between the anode electrode and the cathode electrode is expanded, a withstand voltage is able to be ensured, while the on-resistance increases.
On the contrary, according to the Schottky barrier diode disclosed in the above mentioned document 3, using the two-dimensional electron gas formed at the heterojunction between the GaN layer and the AlGaN layer, the current is flowed laterally. Hence, as similar to the element according to the document 2, the increase of the on-resistance is able to be suppressed.
However, according to the Schottky barrier diode, as similar to the document 2, for improving the withstand voltage, the on-resistance becomes also high. Moreover, as both the ohmic electrode and the Schottky electrode are formed on the same surface, substantially the element area of the diode is not able to be smaller.
Therefore, regarding a Schottky barrier diode, it is preferable to adopt a structure in which the Shottky electrode and the ohmic electrode are substantially arranged in vertical direction. However, as disclosed in the document 1, the thickness of the GaN material formed on the silicon substrate is approximately 850 nm at most, even including the buffer layer. Hence, the withstand voltage becomes hard to be improved. For example, for manufacturing a device having a breakdown voltage of approximately 1200 (V) for electric vehicles, the GaN layer becomes necessary to be a thickness of approximately 10 μm.
On the contrary, a thick formed GaN substrate may be used, however, such a thick substrate is expensive. And then a device using the GaN substrate having a thickness of approximate 10 μm is hard to be popularized at a low price.
Moreover, according to the Schottky barrier diode disclosed in the above mentioned document 2, because of the two-dimensional electron gas, the on-resistance (a series resistance in operation) becomes lower. However, as the Schottky barrier diode is a lateral type device in which the current is flowed laterally, the element area of the device becomes large. Furthermore, when the distance between the anode electrode and the cathode electrode are expanded, the withstand voltage is able to be ensured, while the on-resistance increases.
Therefore, regarding the Schottky barrier diode, for combining the high breakdown voltage with the low on-resistance, it is preferable to adopt a vertical type device in which the Shottky electrode and the ohmic electrode are substantially vertically arranged and the current is flowed vertically. However, the sapphire substrate is not able to be applied to the vertical type device, due to the nonconductive property.
On the contrary, the Schottky barrier diode disclosed in the document 1 is a vertical type device using a silicon substrate, while it has the following problems.    1. Due to the existence of the buffer layer including the AlN layer between the silicon substrate and the GaN layer, a semiconductor layer having a wide band gap exists in the current path. Hence, the series resistance becomes high. For avoiding such the problem, it can be considered to form the buffer layer thinner. However, because the AlN and the AlGaN are wide band gap semiconductors as close to insulators, even when the buffer layer is formed thinner, there is a limit for lowering the resistance.    2. For example, in order to manufacture a high withstand device having a breakdown voltage of approximately 1200 (V) for electric vehicles, GaN layer as a carrier moving layer is necessary to be a thickness of approximately 10 μm. However, the thickness of the GaN layer disclosed in the document 2 is 850 nm at most, even including the buffer layer. Therefore, it is not sufficient to improve the withstand voltage.    3. It is considered that the withstand voltage is able to be increased without increasing the on-resistance, by forming the GaN layer simply thicker. However, as there is a difference between the GaN and the silicon on a coefficient of thermal expansion and a lattice constant, a thick GaN layer having few crystal defects is hard to be grown on the silicon substrate. Moreover, a crack may be occurred on the GaN layer, or a bending or a cracking may be occurred on the silicon substrate. Therefore, these become cause on a device characteristic getting deteriorated and on a yield getting decreased.
Corresponding to the above mentioned problems of 2 and 3, according to the above mentioned document 4 and 5, the electro-conductive GaN layer is adopted. Hence, the energizing in the vertical direction becomes possible. Moreover, thick GaN layer having few crystal defects is able to be epitaxially grown. However, the GaN substrate itself is quite expensive, comparing to such as the silicon substrate or the like. And then as a manufactured product, it is not practical.
Furthermore, the Shottky barrier diode is widely used as a switching power source. And then the high breakdown voltage and the low on-resistance are required therefor. Regarding the conventional Schottky barrier diode using a silicon (Si) material, for realizing the high breakdown voltage, not only a drift layer, in which a depletion layer becomes expanded under a reverse bias, is necessary to be thicker, but also a carrier density is necessary to be low. However, for realizing the decreasing of the on-resistance, under a forward bias, not only the drift layer, in which electrons pass through, is necessary to be thinner, and also the carrier density is necessary to be high. Therefore, in the Schottky barrier diode using Si-based materials, it is difficult to realize both of the high breakdown voltage and the low on-resistance together.
Here, as a gallium nitride (GaN) semiconductor has a high breakdown voltage, even at the time of forming the drift layer thinner, a high withstand voltage is able to be obtained. Therefore, in recent years, for a Schottky barrier diode to be able to realize both of the high withstand voltage and the low on-resistance, the Schottky barrier diode using GaN semiconductor has attracted attention.
For the Schottky barrier diode using such the GaN semiconductor, a Schottky burrier diode 310 shown in FIG. 21 is disclosed in the above mentioned document 1. The Schottky burrier diode 310 comprises a buffer layer 303 formed using an aluminum nitride (AlN) or a Gallium nitride (GaN) on a Si substrate 304, a GaN drift layer 302 formed using a GaN semiconductor, a Schottky (an anode) electrode 301, and an ohmic (a cathode) electrode 305 formed on the other side of the Si substrate 304. In the Schottky barrier diode 310, when a reverse bias is applied, a depletion layer becomes expanded in the GaN drift layer 302. Thus, a high breakdown voltage becomes possible to be realized. Furthermore, when a forward bias is applied, electrons are flowed from the ohmic electrode 305 through the GaN drift layer 302 to the Schottky electrode 301.
Moreover, for the Schottky barrier diode using such the GaN semiconductor, a Schottky burrier diode 410 shown in FIG. 22 is disclosed in the above mentioned document 3. The Schottky burrier diode 410 comprises a buffer layer 403 formed using an AlN on a Si substrate 402, a first semiconductor layer 404 formed using a GaN semiconductor, a second semiconductor layer 405 formed using an aluminum gallium nitride (AlGaN), a Schottky electrode 406, and a rear electrode 401 formed on the other side of the Si substrate 402. Furthermore, the Schottky barrier diode 410 comprises an ohmic electrode 407 formed on the same surface as the Schottky electrode 406 and in a via 408 which passes through the first semiconductor layer 404 and the buffer layer 403 to reach the Si substrate 402.
In the Schottky barrier diode 410, when a forward bias is applied, a current is flowed from the Schottky electrode 406 to the ohmic electrode 407, because of a two-dimensional electron gas formed at an interface between the first semiconductor layer 404 and the second semiconductor layer 405. And then, the current is flowed from the ohmic electrode 407 through the via 408 and the Si substrate 402 to the rear electrode 401. Moreover, when a reverse bias is applied between the Schottky electrode 406 and the rear electrode 401, a depletion layer becomes expanded in a region under the Schottky electrode 406 and between the first semiconductor layer 404 and the second semiconductor layer 405. Hence, the current is not flowed between the Schottky electrode 406 and the rear electrode 401. Thus, the high withstand voltage becomes possible to be realized.
However, in the conventional Schottky barrier diode 310, for relaxing a strain caused by a difference of lattice constants or coefficients of thermal expansion between the Si substrate and GaN, the buffer layer 303 including the AlN layer is formed. However, as the AlN layer includes plenty of defects and has a high resistance, a sufficient current is not able to be flowed in the vertical direction in FIG. 21. Hence, the on-resistance of the Schottky barrier diode 310 is not able to be lowered.
Moreover, in the conventional Schottky barrier diode 410, as both of the Schottky electrode 406 and the ohmic electrode 407 are formed on the same surface, a chip size becomes large corresponding to an area of the ohmic electrode 407. Furthermore, in the Schottky barrier diode 410, a withstand voltage is determined based on a distance between the Schottky electrode 406 and the ohmic electrode 407. Therefore, in the Schottky barrier diode 410, for obtaining a Schottky barrier diode of a high withstand voltage, the distance between the Schottky electrode 406 and the ohmic electrode 407 is necessary to be long. Hence, the chip size becomes larger.